KRT19 and colitis: Both PC cluster 1 and cluster 2 are characterized by increased expression of two markers of radio-resistant and secretory progenitors with self-renewal capacity during regeneration, namely Krt19 (ref. 24) and Atoh1 (ref. 25), whereas increased expression of Reg3b, known for its protective role in the development of colitis and ileitis26, and Cdkn1a (encoding p21), a marker of terminal differentiation in the intestine27, was observed in cluster 2 compared with cluster 1.